The present invention is generally directed to a current sense circuit and, more specifically, to a current sense circuit for use with an electro-optic element, such as an electrochromic element.
In various electronic circuits, it may be necessary or advantageous to monitor a current delivered by a drive circuit to a load so as to protect the load from over-current conditions. When a drive circuit receives its operating power from a power source whose output varies, e.g., an unregulated power supply, a drive current supplied by the drive circuit to a load will vary depending upon the voltage level of the power source. In situations where a series resistor is placed between a drive circuit and a load, to monitor the drive current supplied to the load, a monitoring circuit that monitors the voltage across the series resistor (to determine the current delivered to the load) may be unable to detect if the load is shorted when the output voltage level of the power source is at its lower extreme. This is due to the fact that circuits that have been used to monitor the voltage across the series resistor have had a fixed threshold, which has been set above a desired operating current. However, if the fixed threshold is set to a lower level, such as the current level that would be delivered to a load at the lower extreme output of a power source, false over-current indications may occur during normal operation.
As an example, in electrochromic (EC) vehicular rearview mirror assemblies, a drive circuit is provided, which varies the voltage across an EC element to vary the reflectivity of the mirror such that unwanted glare can be avoided or reduced. However, it is not uncommon for the power source that supplies the drive current to the drive circuit to vary from, for example, between nine and sixteen volts. As previously mentioned, when a series resistor is utilized for monitoring the current delivered to the load by the drive circuit, setting a fixed threshold to a nominal voltage, e.g., twelve and one-half volts, can prevent the monitoring circuit from detecting short circuits at the load at the lower voltage extreme when the fixed threshold is set above the value that can be achieved at the lower voltage.
Thus, what is needed is a current sense circuit with a variable threshold that is capable of tracking variations in a power source output level.
An embodiment of the present invention is directed to a current sense circuit that includes a current sense device and a voltage sense device. The current sense device is positioned to sense a drive current provided to a load by a drive circuit. The voltage sense device is coupled across the current sense device and receives a variable threshold signal at a first input and provides an output signal on an output whose value is dependent upon whether a sense signal at a second input is above or below the variable threshold signal whose level changes in response to a voltage level of a power supply that supplies the drive current to the drive circuit. In one embodiment, the load is an electrochromic element. In another embodiment, the current sense device is a resistor. In still another embodiment, the voltage sense device is a differential amplifier. In another embodiment, the output of the voltage sense device is coupled to an input of a control unit that controls a level of the drive current provided by the drive circuit in response to the output signal.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.